While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate at a much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Appl. Phys. Vol. 65, Pages 3610-3616, 1989]. Furthermore, in some preferred device structures, the LEL is constructed of a doped organic film comprising an organic material as the host and a small concentration of a fluorescent compound as the dopant. Improvements in EL efficiency, chromaticity, and lifetime have been obtained in these doped OLED devices by selecting an appropriate dopant-host composition. The dopant, being the dominant emissive center, is selected to produce the desirable EL colors. There are known examples of green dopants. Green emitting coumarin derivatives have been reported by Chen et al. in U.S. Pat. No. 6,020,078, Inoe et al. in JP 3142378, and Tang et al. in U.S. Pat. No. 4,769,292. While coumarin derivatives have been shown to provide OLED devices with high EL efficiencies, it has been found that a long device operational lifetime is lacking. Green emitting quinacridone derivatives have been reported by Shi et al. in U.S. Pat. No. 5,593,788 and Tamano et al. in JP 3509383. While it has been shown that quinacridone derivatives provide a long operational life, use of these derivatives results in lower device efficiencies.
Aminoanthracene derivatives have been shown to be useful in OLEDs. Enokida et al., in U.S. Pat. No. 6,251,531, and Matsuura and co-workers in US2006/0033421 and US2005/0064233 describe certain 9,10 substituted aminoanthracenes for use as light-emitting materials, however these types of materials have generally been shown to have poor lifetimes, low efficiencies or they emit in the yellow-green to yellow regions of the visible spectrum, meaning that the CIE x coordinate is above 0.30.
Ichinosawa et al., in JP 2003/146951 and JP 2004/091334, describe anthracene materials substituted with phenylene diamine groups in the 2,6 positions that are useful as hole-transporting materials for EL devices and provide examples of their use in a layer adjacent to the LEL.
Toshio et al., in JP 1995/109449, provides examples of anthracene-type materials substituted with tertiary amine groups and their use as light-emitting materials without being incorporated into an LEL host material. It is known that having a single component LEL generally results in low efficiencies and short lifetimes.
Hosokawa and co-workers, in US 2003/0072966, US 2005/0038296, U.S. Pat. No. 6,951,693 and Ikeda et al., in JP 2003/313156 also describe certain tertiary amino-anthracene compounds for use in OLED devices. However, reported luminous efficiencies are low and device lifetimes are poor.
However, despite these advances in the research of green emitting materials, there is still a need for green OLED devices that combine high luminous efficiency with long operational lifetime while maintaining excellent color purity.